This invention relates to coated type magnetic recording media, and more particularly, to an improvement in the composition of particulate magnetic material used in a magnetic layer of magnetic recording media. The magnetic recording media to which this invention pertains include magnetic recording tapes for audio and video applications and magnetic sheets or disks to be loaded in a magnetic sheet operating device such as an electronic or electromagnetic camera for recording magnetic signals on the sheet.
In these years, a variety of magnetic articles of manufacture have been commercially utilized including high performance audio cassette tape, video tape, computer tape, multi-coated tape, magnetic disks, floppy disks, magnetic cards, and the like as symbolically represented by the commercial success of home VCR units. Active work has been addressed to the development of new particulate magnetic materials useful for such magnetic recording media.
One typical example of particulate magnetic material is needle or acicular iron oxide. The needle iron oxide alone, however, is insufficient to accommodate the need for higher magnetic recording density. Instead, cobalt-coated iron oxide and magnetic metal particles were proposed and have been used in practice in some high performance audio cassette tape, video tape, and various magnetic disks. The majority of video and audio tapes, however, use needle iron oxide. Their problems will be further described.
Needle iron oxide has insufficient magnetic properties including coercive force Hc and maximum saturation magnetization Ms, failing to provide satisfactory output and SN ratio. In order that a magnetic coating have an increased residual magnetization Br, the coating composition must have such a higher content of needle iron oxide powder as to render the coating less durable. A magnetic layer containing needle iron oxide has a high electric resistance and is liable to such inconvenience as dropout. Addition of an antistatic agent such as carbon to a magnetic layer can reduce the electric resistance of the layer at the sacrifice of the residual magnetization Br, output, and SN ratio of the layer.
Cobalt-coated iron oxide particles have insufficient magnetic properties as exemplified by a low maximum saturation magnetization Ms and easy demagnetization. They have an electric resistance insufficient to prevent dropout increase during repeated operation and are less durable.
Magnetic metal particles also have many problems including low abrasion resistance, marked deterioration during service, and oxidation susceptibility. Even after incorporated in a magnetic recording medium, there still occurs a reduction of magnetic flux density, resulting in an output drop.
When a magnetic recording medium having magnetic metal particles is operated across a magnetic head of Sendust or amorphous metal material, there often occurs so-called "seizing". That is, a lusterless discolored layer is formed on the head surface, the layer being of different color from the remaining head material. It is believed that the seizing is a chemical and physical quality change of Sendust or amorphous metal material on the surface due to the sliding contact between the magnetic recording medium and the magnetic head.
Another problem is associated with a process of forming magnetic recording media using magnetic metal particles. It is imperative that an oxide coating is formed on the surface of magnetic metal particles during handling. As the case may be, an oxide coating is intentionally formed.
Magnetic recording media using magnetic metal particles having an oxide coating formed on the surface thereof have the advantages that they do not undergo a reduction in magnetic flux density due to the influence of ambient conditions, for example, temperature and humidity or a deterioration in properties due to rust formation in the magnetic layer. However, since the surface oxide coating provides an increased electrical resistance, the magnetic recording medium tends to be electrically charged at the surface so that foreign matter will adhere to the surface, causing dropouts. Further, electric charges upon exfoliating cause discharge noises. All these factors detract from tape performance.
Magnetic sheets capable of recording magnetic signals representative of a still image in an annular track are known in the art. It is a common practice to accommodate such a magnetic sheet in a jacket for convenience of handling and storage. The magnetic sheet comprises a flexible substrate and a magnetic layer formed thereon. Usually, a pair of magnetic layers are coated on the opposed surfaces of the substrate partially for the purpose of preventing the sheet from warping.
On use, a magnetic sheet is mounted on a drive shaft of a magnetic sheet operating device and rotated at a high speed, for example, at 3,600 rpm. While the sheet is being rotated, a magnetic head is forced against the sheet to perform recording and reproducing operations on the sheet. A stabilizer member holds the opposite surface of the sheet to control the degree of contact between the sheet and the head. In this way, recording/reproducing operation is performed on the magnetic sheet in an electronic camera while the sheet is rotated at a high speed and pressed with a sufficient force to induce deformation. Thus the magnetic sheet must be highly durable.
For the same reason, an output of reproduction from the magnetic sheet somewhat depends on the relative location of the head and the quantity of deformation of the sheet. Output varies with the degree of contact between the head and the sheet. Thus the magnetic sheet must also have appropriate rigidity or stiffness. Because of the above-mentioned operation mode, the magnetic sheet simultaneously undergoes high speed sliding motion by rotation and deformation by bending. That is, the magnetic sheet experiences a unique mechanical fatigue of different type from those of conventional magnetic tape and disk.
There is a strong need for a magnetic sheet satisfying these considerations.